Semi-permeable membranes become fouled spontaneously during normal operation process and conditions through the accumulation of fouling media. A fouled membrane has a reduced separability of the dissolved salts, a reduced flux rate, an increased pressure loss and, therefore, has to be cleaned.
Fouling media includes minerals and organic particles, scaling microcrystals, bacteria, and algae. All these components are contained in a biofilm matrix made by bacteria. There are no free particles on the membrane surface. All the fouling elements are interconnected by the biofilm. Therefore, fouling is characterized as a layer of interconnected elements laying on a membrane surface as a carpet. In the biofilm matrix exists the most concentrated part of feed solution, which is called and causes Concentration Polarization. In this invention, the term “fouling” includes all of the different forms of fouling as mentioned above or known to the skilled man in the art.
The fouling may be located on the feed or the permeate membrane sides. However, most of the fouling is located on the feed membrane side. Due to the membrane structure, the thickness of the fouling biofilm is not equal over the entire surface of the membrane.
A permeate spacer is a grid of solid fibers, defining semi-rectangular void spaces. The permeate spacer is located between the permeate sides of two opposing membrane layers defining a permeate channel. On top of each of such a semi-rectangular void space, there is a free membrane layer portion which may stretch down and displace toward the permeate side, a phenomena called sagging toward permeate side. This displacement takes place because the gauge pressure on the feed side is higher than the gauge pressure on the permeate side. As a result, under these circumstances the membrane surface gets a wave-like profile in which valleys caused by sagging are located above the void spaces of the permeate spacer and the hills are located on top of its fibers. Similarly, a feed spacer is usually located between two opposing feed sides of two opposing membrane layers defining a feed channel. The feed spacer is also a mesh of solid fibers which define void spaces. Typically, the void spaces of the feed spacer are bigger than the void spaces of the permeate spacer. In the following description “displacement toward the feed side” or “feed forward sagging” means that the displacement toward the permeate side is diminished. In the following description, a membrane may oscillate between a more displaced position (sagged) toward the permeate side and a less displaced position (sagged) toward the permeate side. In a spiral membrane, this wave-like surface dictates a feed flow regime along and across the membrane which enhances the accumulation of more fouling media in the valleys areas than in the hills areas. In these valleys areas the longitudinal flow velocity is lower and more fouling media tends to accumulate.
Standard methods for membrane cleaning involve: stopping the RO desalination process; disconnecting membrane modules from the high pressure pump; connecting the membrane modules to a Cleaning In Place (CIP) unit; pumping by a low gauge pressure harsh chemicals along the feed membrane side. This CIP method is expensive, not effective, and creates environmental problems.
U.S. Pat. No. 4,952,317, describes separating colloidal suspension by non directional, tangential vibration of the membrane elements. This vibration induces the creation of intensive shear forces between the membrane and the colloidal suspension. Such a vibration treats and vibrates the membrane as a whole and does not distinguish between membrane and spacers or between the feed channel and the permeate channel.
This technique has to be applied continuously during the normal operation of the membrane. However, continues vibration application consumes significant power, and accelerates membrane wear and tear. However, as explained above the nature of the fouling in most of the cases is not a colloidal suspension nature, but rather a biofilm matrix behaves as carpet/layer nature which tends to be attached to the membrane by adhesion forces. Tangential vibrating of the membrane elements having such a carpet layer of fouling adhered to the membrane surface cannot remove it. Therefore, it is one aspect of the present invention to provide a cleaning solution to remove a fouling layer characterized by a carpet structure.
A known RO membrane Cleaning Method is disclosed in WO/2005/123232, also known as “DOHS process”. This DOHS process teaches the implementation of a cleaning method based on a temporarily inline injection of a Super Saline Solution having a high osmotic pressure into the feed line, so that the Net Driving Pressure across the membrane is locally and temporarily reversed. As a result, the osmotic process over a membrane area switches from a reverse osmosis process to a direct osmosis process. Once the Super Saline Solution propagates downstream the membrane module, then the local osmotic pressure regime across the membrane is restored to its original values and the system then switches back to a reverse osmosis process. This procedure is done without stopping the desalination unit, without releasing the gauge pressures in the system and without inducing membrane sagging. It is mainly the osmotic pressure which is being changed and this cannot cause membrane sagging. The invention further teaches that switching from a reverse osmosis process to a direct osmosis process provides intensive permeate backward flow of permeate into the Raw Saline solution. The local backward flow of permeate from the permeate side into the feed-brine side may clean the membrane.
Additionally, several patents and patent applications implement gauge pressure pulsation for membrane cleaning, such as US2011/0315612; U.S. Pat. No. 7,658,852 (B2); U.S. Pat. No. 5,690,829 (A); US2012318737 (A1); JP 2005238135 (A); U.S. Pat. No. 3,853,756 (A); U.S. Pat. No. 7,097,769 (B2).
The intention of gauge pressure pulsation is to change the process from reverse osmosis to forward osmosis. Generally, a pulse generator is positioned in the feed stream and most of them implement high amplitude of pressure pulsation, such as 30-60 bar. In permeate lines such big amplitude request implement expensive pipe material and assembly work. Such a large amplitude of pressure change can be done by gauge pressure changing velocity of not more than 10 psi per second. This means that expected frequency can be very small.